Attribution is not required, but it is encouraged.
If you want to show some support for this (free!) service, place a notice like this somewhere:
Thank you to <a href="https://crafatar.com">Crafatar</a> for providing avatars.

You can tweak images using query string parameters.
Example: https://crafatar.com/avatars/853c80ef3c3749fdaa49938b674adae6?size=4&default=MHF_Steve&overlay

size: The size for avatars in pixels. 1 - 512

scale: The scale factor for renders. 1 - 10

overlay: Apply the overlay to the avatar. Presence of this parameter implies true. This option was previously known as helm.

default: The fallback to be used when the requested image cannot be served. You can use a custom URL, any uuid, or MHF_Steve/MHF_Alex.
The option defaults to either MHF_Steve or MHF_Alex, depending on Minecraft's default for the requested UUID.

Crafatar checks for skin updates every 20 minutes.
Images are cached in your browser for 60 minutes until a new request to Crafatar is made.
In addition, CloudFlare caches up to 2 hours on a per-url basis.

When you changed your skin you can try clearing your browser cache to see the change faster.